1. Field of the Invention
The present invention relates generally to RF circuits, and particularly to RF circuits using ceramic substrates.
2. Technical Background
A stripline circuit is used in RF and microwave circuit applications and is implemented by disposing a transmission line between two ground planes. A dielectric material is disposed between the transmission line conductor and each of the ground planes. Stripline structures are typically employed in the manufacture of directional couplers, baluns, power dividers and other such devices used in RF, microwave and millimeter wave circuits. Currently there are two industry accepted methods for manufacturing stripline transmission lines and RF/Microwave filters. The first method uses commercially available copper clad laminate materials, whereas the second is based on low temperature co-fired ceramic (LTCC) construction.
With respect to the first method, commercially available copper clad laminates are often used for stripline applications. Copper clad laminates are composite materials typically made by mixing some combination of PTFE, glass, and ceramic materials in accordance with the manufacturer's specific proprietary blend. Stated differently, the manufacturer can tailor the mechanical and electrical properties of the laminate material by varying the mixture ingredients and their amounts. Once the laminate sheet has been produced, it is then clad with copper. Most copper clad laminate materials are mechanically flexible such that they can easily survive the relatively extreme temperatures and pressures associated with the production of multi-layer stripline circuits. In addition, copper clad laminate materials allow for the production of highly complex assemblies that have multiple layers. Moreover, these materials may be used to implement circuits that can operate at very high frequencies. However, there are drawbacks associated with using these laminates for RF, microwave or millimeter wave filters and the like.
One drawback relates to the dielectric loss of laminate materials. Compared to an alumina ceramic material (such as, e.g., the Coors Tek ADS996 material), the dielectric loss can be as much as forty times that of an alumina substrate. In addition, the manufacturing tolerances associated with copper clad laminates are also inferior to that of alumina ceramic materials. For example, the dielectric constant variation of a copper clad laminate can be as much as double that of an alumina substrate. Moreover, the metallization etching tolerances associated with laminate materials can be about five times that possible on an alumina substrate such as the ADS996 material. Finally, the laminate materials have a tendency to warp and stretch during the processing and lamination steps because they are mechanically soft. This warping and stretching introduces alignment errors between etched copper circuit features of the filter and the “drilled-hole” circuit features of the stripline filter. These alignment errors can be as much as five times that possibly with ceramics such as ADS996. All of the aforementioned drawbacks add cost by complicating the filter design and reduce the manufacturing yield of the filter, in addition to degrading the filter's performance.
The second industry accepted method for manufacturing stripline transmission lines and RF/Microwave filters employs low temperature co-fired ceramics (LTCC). LTCC is a process in which a cast ceramic, or “green tape”, comprised of ceramic powder and binder is used. The green tape is processed prior to being fired and is thus mechanically soft. In order to create a stripline filter, a metal transmission line layer is screen printed onto the green tape. Additional layers of green tape are placed overtop the metal layer and the assembly is co-fired to complete the process.
There are also drawbacks to the second method (LTCC) process. First of all, the co-firing process, in which both the metallic paste and cast ceramic are fired, causes the entire assembly to shrink somewhat. Much work has been done to predict and control this shrinking, but it still introduces alignment errors between internal metallic layers of the filter and the punched hole features of the filter. Likewise, the copper clad laminate materials used in the first process, the green tape is a composite material that contains both ceramic and binders; thus, the dielectric loss parameters can be as large as sixty times that of alumina materials (See, e.g., the Coors Tek ADS996 material).
In yet another approach, a stripline filter structure employing pre-fired ceramic materials in a laminate structure has been considered. In this approach, two pre-fired ceramic substrates are used to fabricate a stripline structure. The metallic transmission line structure is printed on a surface of one of the pre-fired ceramic layers such that it is disposed between the two substrates. During this process, multiple layers of a glass sealant material are deposited between the pre-fired ceramic layers until a desired thickness is obtained. The thickness is a function of the desired operating frequency of the stripline structure. The glass layers are heated to join the two pre-fired ceramic layers together to create the stripline structure. While the use of pre-fired ceramic layers obviates some of the issues described above, there are drawbacks associated with the process and the resultant stripline product. For example, the application of multiple glass layers requires too many processing steps that result in an increase in processing time. Moreover, the addition of the glass layer prohibits the use of conductive via holes between the center conductor of the stripline and outer ground conductors. These via holes are required for the stripline circuit to function properly. In addition, the resulting stripline structure often experiences tolerance issues that cannot be overcome. As a result, many of the devices are ultimately rejected and deemed to be waste. For all of these reasons, the last approach discussed herein is costly and inefficient.
What is needed therefore is a stripline filter structure that substantially addresses the needs described above. There is a need for a stripline structure that efficiently uses pre-fired ceramic materials and exhibits a dielectric loss parameter that is significantly lower than that possible with either copper clad laminates or LTCC. There is also a need for a stripline structure that exhibits improved manufacturing tolerances vis á vis all of the methods described above.